Electric heating element



March 17 1942. LAUBMEYER 2,276,583

ELECTRI C HEATING ELEMENT Fi led June 23, 1939 zznik er .Zaabm eye-7'RQM A5 to rneya' Inven Zar:

- metals, which core is resistant Patented Mar. 17, 1942 PATENT OFFICEELECTRIC HEATING ELEMENT Giinther Laubmeyer, Kassel-Wilhelmshohe,

Germany Application June 23, 1939, Serial No. 280,885 In Germany June30, 1938 Claims.

This invention is directed to an electrical re sistance heating element.More particularly, the invention is directed to the construction of theelectrical heating element of a material not readily oxidized.

Electrical resistance heating elements adapted to operate under heavyload at high temperatures have generally heretofore been constructedupon a core of substantially pure nickel, -or nickel-steel alloys, orother expensive steels, such as V2A nickel-chromium steel. Because ofthe high temperatures developed in the heating elements, ordinaryinexpensive metals such as iron oxidize to such an extent that thelayers of oxidized material developed upon the core eventually build upto the point where the electrical conductors break under pressure andconsequently the heating element fails.

The art has long searched for a substitute for the expensive pure nickelused to form the cores of the resistance elements. Nickel-steel alloysare not satisfactory as they are likewise expensive and are, moreover,difficult to work into desired shapes. Heretofore, a satisfactory coremade of cheap, easily worked material has not 5 been discovered.

It is an object of this invention to construct a heating element havinga core made of low cost to oxidation under high temperatures.

3 Another object of the invention is to find a satisfactory substitutefor the expensive pure nickel and alloy nickel-steel cores used formaking high temperature electrical resistance heating units.

Another object of the invention is to produce a resistance heatingelement having either a circularly wound or a flatly wound resistancecoil supported upon a core of cheap material not greatly oxidized athigh temperatures.

Generally these objects are obtained by constructing an iron core havingan aluminum alloy surface produced by diffusion welding.

Both aluminum and iron are relatively cheap of aluminum around theironcore, and then producing a diffusion welding between the aluminum andthe surface of the iron core by completing the resistance element andbringing it to a temperature at which the diii'uson be- The means bywhich the objects of the invention are obtained may be more fullyunderstood with reference to the accompanying drawing, in which:

Fig. 1 is a cross view of a heating element constructed according tothis invention.

Fig. 2 is a cross sectional view of the core having the layers ofaluminum applied thereto.

Fig. 3 is a cross sectional view on the line 3-3 of Fig. 2'.

Fig. 4 is a view corresponding to Fig. 3, bu showing the aluminum layerafter diffusion has taken place between the aluminum and the iron.

Fig. 5 is a cross sectional view of the core with the aluminum layerssuch as shown in Fig. 3 inserted within a mold.

In Fig. 1 the iron core I0 is coated with an aluminum layer l2. The ironcore is shown as a tube of circular section with outwardly flared 0ends. Obviously tube It may be of oval, or

polygonal, section, and may be solid. The surface of the core III iscleaned of oxide by ordinary methods, such as brushing, grinding, orscouring.

The aluminum layer I2 can be applied in the form of thin flexiblesheets. or layers of foil. A plurality of layers are usually applied,inasmuch as under long, continual heating, the aluminum eventuallybecomes entirely diffused throughout the iron, and the greater thequantity of alumi-.

num initially applied means that the life of the unit will be longer.The aluminum layers are tightly wound about the core 10.

The electrical conductors l8 are wound about materials, especially whencompared with exthe alummmn coated core. these wires being pensivechrome or nickel-steel alloys, and boththe iron and aluminum are, bythemselves, ordinarily unusable for the formation of high temperatureheating elements. Iron by itself oxidizes excessively at hightemperatures, and

aluminum by itself is too soft. However, an aluminum iron alloy hasstrength and low oxidation characteristics at high temperatures andconsequently produce a satisfactory mateseparated by mica insulation ID.The electrical leads to the coils 16 are shown at 20 and 22. Alongitudinally split sheet metal sleeve 24 covers the coils l6. Thissleeve is wrapped by a steel wire 26. Coil l6 and the externally wrappedwire 28 both aid in pressing the aluminum layer I! very tightly againstthe surface of core I.

The heating element thus constructed is then rial. The alloy can beobtained by forming layers heated through the medium of the resistanceof the coil It to an electric current up to or slightly more than themelting point of the aluminum, and at this temperature diffusion takesplace between the aluminum and the iron, this diffusion being aided bythe externally applied pressure upon the aluminum against the surface ofthe core, as well as by the expansion of core Ill during the heating. Analuminum iron alloy H is thus formed on the exterior of the core, thealloy having a high aluminum content on the outside which graduallydecreases toward the interior of the core l0. As aluminum iron alloysare highly refractory, the surface of the core is protected againstoxidation at high temperatures, and consequently the heating element isensured of a long life. A satisfactory substitute for the expensivenickel steels is thus achieved.

In Fig. 5, a second method is shown of producing the core with analuminum iron alloy surface. The 'core, with the layers of aluminumthereon as seen in Fig. 3, is inserted between the two halves 28 and 30of a mold. The halves of the mold are tightened upon the aluminumcovered core by means of bolts 32 and nuts 34 and 36, the aluminumcoating thus being pressed very tightly against the surface of the core.The mold is then placed in a furnace and the temperature of the core andaluminum raised to the point where the aluminum diffuses with the ironof the core to become alloyed therewith. An insulating layer 38 of micais inserted between thealuminum l2 and the halves of the core so that nodiffusion takes place between the aluminum and the mold.

Instead of a mold such as shown in Fig. 5, a metal sleeve or band' couldbe employed,'said sleeve or band being insulated from the aluminum bymeans of mica or asbestos. The metal band may be wound about thealuminum coated core steel has been obtained. Use has been made of tworelatively cheap materials by causing them to be alloyed, and thealloying of the metal can take place as one of the steps in the normalassembly of the elements of the heating unit.

Having now described a means by which the objects of this invention maybe obtained, what I claim as new and desire to secure by Letters Patentis:

1. An electrical heating resistance unit comprising an iron core, atleast one layer composed of aluminum in sheet form placed on said core,

and electrical heating elements firmly contacting said layer to holdsaid layer to said core whereby an iron-aluminum alloy zone is formedbetween said core and said layer when said unit is heated during use.

2. An electrical heating resistance unit comprising an iron core, asheet of aluminum enclosing said core, and insulated electrical heatingcoils tightly wound upon said sheet to fasten it to said core whereby aniron-aluminum alloy zone is formed between said core and said sheet whensaid unit is heated during use.

3. An electrical heating resistance unit comprising an iron core; atleast one layer composed of an aluminum sheet upon the surface oi. saidcore, insulated electrical heating elements contacting said layer, asleeve enclosing said eleand after being so wound, tightened so as topress the aluminum tightly against the core. After the core has thusbeen prepared as shown in Fig. 5, the heating elements Is, as shown inFig. 1, are applied to the aluminum surface core in order to completethe heating unit.

As has been stated, the core can assume any cross sectional form, andcan be either hollow or solid. Likewise, the electrical conductors may,

be either cylindrically coiled as shown, or may be flatly formed. In anycase, an aluminum iron alloy is formed upon the surface of the core sothat layers of oxidized iron will not occur during the use of theheating element under heavy loads and high temperatures.

merits, and means for tightening said sleeve upon said elements to holdsaid layer in firm contact with said core whereby an iron-aluminum alloyzone is formed between said core and said layer when said unit is heatedduring use.

4. An electrical heating resistance unit comprising an iron core, atleast one layer of aluminum foil upon the surface of said core, and aheating element tightly securing said aluminum layer to said corewhereby heating of said unit produces by diflfusion an iron-aluminumsurface upon said core.

5. An electrical heating resistance unit comprising an iron core, aplurality of layers of sheet aluminum upon said core, insulatedelectrical heating elements upon said layers, and means for securing andtightly pressing said elements and layers tightly against said corewhereby an iron-aluminum alloy zone is formed between said core and saidlayer when said unit is heated

